1. Field of the Invention
The present invention relates to sonic logging in a borehole traversing an earth formation, and particularly to logging compressional, shear and Stoneley modes of sonic propagation in a single logging operation.
2. Background Information
One type of existing sonic tool used in borehole logging employs a monopole sending transducer for transmitting pulses of sonic energy covering a broad frequency bandwidth which induce propagation of sonic waves in the borehole and surrounding formation. Energy of the sonic waves is detected at each of an array of monopole receiver transducers to produce a set of received signals. A "first motion" detection method is applied if only the formation compressional wave velocities are to be measured, or the entire time-domain waveforms of the set of received signals are recorded if data are needed for compressional, shear and Stoneley waves.
A logging tool of this type is described, for example, in C. MORRIS et al., A New Sonic Array Tool for Full Waveform Logging, SPE Paper 13285, presented at the 59th Annual Technical Conference and Exhibit of the SPE, Houston, Tex., 16-19 September 1984. This paper describes a monopole array sonic waveform logging tool and processing procedures for compressional waves, shear waves (in "fast" formations, i.e., hard rocks), and Stoneley waves. Full waveforms are recorded in time domain, as shown in the examples of FIG. 4 of the paper. Compressional (P), shear (S), and Stoneley slowness logs are obtained by processing the time-domain waveforms.
An example of a monopole sonic source transducer intended for generating compressional as well as shear waves is described in U.S. Pat. No. 4,383,591 to Ogura. Time-domain recording of full waveforms is employed.
Monopole tools have not proven entirely satisfactory for acquisition of formation shear data, especially since the shear log can be missing in certain regions of the borehole due to "slow" formations. (A "slow" formation has a shear velocity less than the borehole fluid compressional velocity.) Accordingly, a second type of sonic tool, a dipole shear tool, is added to the monopole sonic tool. Dipole shear tools employ dipole source and dipole receiver transducers (rather than monopole source and monopole receiver transducers) for direct logging of shear in both "fast" and "slow" formations. Pulses of sonic energy covering a suitable frequency bandwidth are transmitted by the dipole source transducer to induce propagation of shear waves in the formation, and energy of the shear waves is detected at each of an array of dipole receiver transducers to produce a set of received dipole signals. The entire time-domain waveforms of the set of received dipole signals are recorded.
One approach to logging P and S waves simultaneously in unconsolidated (soft) rocks is described in U.S. Pat. No. 4,383,308 to Caldwell. Shear-wave data are obtained from time-domain dipole measurements, and compressional-wave data are obtained from time-domain monopole measurements. Because the measurements are made in the time domain, special arrangement of the source-receiver spacings and special source transducer firing sequences are required to measure P and S waves in the same logging run. This method is impractical for simultaneous acquisition because the monopole and dipole waves can sometimes interfere with each other due to imperfect borehole conditions such as tool eccentering and non-circular boreholes. It also limits the choice of source to receiver spacing for each measurement.
Another approach to logging P and S waves simultaneously is disclosed in U.S. Pat. No. 4,516,228 to Zemanek. In this approach, time-domain waveforms are obtained by alternately firing and receiving dipole and monopole waveforms using bimorph bender transducers.
Yet another approach to logging multiple sonic propagation modes with one tool is described in U.S. Pat. No. 3,475,722 to White and in J. WHITE, The Hula Log: A Proposed Acoustic Tool, paper presented at the SPWLA Meeting, 1967. The proposed tool has sets of transducers in contact with the borehole wall which are capable several different types of measurements by generating monopole, dipole and torsional waves to acquire time-domain waveforms. Simultaneous acquisition of the various measurements is not disclosed.
The DSI.TM. tool recently commercialized by Schlumberger combines monopole and dipole transducers in a single tool to log for compressional, shear and Stoneley waves in the borehole. This tool is suitable for logging both soft ("slow") and hard ("fast") formations. The DSI.TM. tool is described, for example, in A. HARRISON et al., Acquisition and Analysis of Sonic Waveforms from a Borehole Monopole and Dipole Source for the Determination of Compressional and Shear Speeds and Their Relation to Rock Mechanical Properties and Surface Seismic Data, SPE Paper 20557, presented at the 65th annual SPE meeting, New Orleans, La., Sep. 23-26, 1990. To get the complete logs, the monopole and dipole transmitters are fired separately at separate times. Large amounts of time-domain waveform data are recorded and transmitted uphole. Signal processing to extract wave velocities from the arrays of waveforms is done uphole, using computers in the logging unit or at offsite computing facilities.
Time domain waveform recording for P, S and Stoneley wave logging, whether monopole or dipole, involves a large amount of data to digitize, transmit, store, and process. For example, the DSI.TM. tool has an array of 8 receivers, and can record dipole waveforms as well as monopole waveforms in high-, low-, and mid-frequency ranges. At least 512 time samples are taken of the waveform from each receiver, each sample being digitized at 12 bit resolution. The total waveform data for one receiver array per depth point in the borehole is thus 8.times.512.times.12/8 bytes, which is about 6 kbytes per depth point.
Following are the data acquisition modes of the DSI.TM. tool, and the corresponding quantities of data acquired per depth point in each mode:
Time-domain Dipole Array Waveforms.apprxeq.6 kbytes/depth-point PA0 Time-domain Monopole Stoneley Waveforms.apprxeq.6 kbytes/depth-point PA0 Time-domain Monopole P & S Waveforms.apprxeq.6 kbytes/depth-point PA0 Digital First-Motion Detection.apprxeq.0.08 kbytes/depth-point PA0 (a) Telemetry Rate and Logging Speed. For example, if the logging speed is 2400 ft/hour and the depth points are at 6-inch intervals, then the telemetry rate needed to handle two time-domain waveform logs (e.g., dipole array waveforms and monopole Stoneley waveforms) is at least 128 kBits/second (2 logs.times.6 kbytes/depth-point.times.8 bits/byte.times.2400 ft/hour.times.2 depth-points/ft.times.1/3600 hr/sec). Even with a high data rate telemetry system, this amount of time-domain waveform data occupies the majority of the telemetry channel capacity. PA0 (b) Data Storage. For example, in a depth section of 10,000 ft with depth-points at 6-inch intervals, there are 20,000 depth points. With the DSI.TM. tool's time domain waveform acquisition, about 240 MBytes of data must be stored and handled for the sonic logs of two waveform acquisition modes in this example. PA0 (c) Processing. Even with the processors available in the Schlumberger's computerized CSU.TM. and MAXIS.TM. logging units, a major portion of the processing capacity can be consumed in handling the time-domain waveforms.
If any two of the waveform acquisition modes are logged simultaneously, the amount of time-domain waveform data that must be sent uphole will total more than 12 kBytes/depth-point. (The data handling requirements for compressional-wave "first motion" detection are more modest than those for full time-domain waveforms. See, for example, the techniques described in U.S. Pat. No. 4,985,873 to Eyl et al.)
Several factors must be considered in handling the acquired time-domain waveform data:
The demands of handling the time-domain waveforms acquired with a logging tool such as the DSI.TM. can limit the logging speed as well as the combinability of the tool with other logging tools. Since rig time is often expensive, logging speed and avoidance of another logging run is an important consideration.